Boston marathon start times 2013 EDT:
9:00 am Mobility Impaired
9:17 am Wheelchair Division
9:22 am Handcycles
9:32 am Elite Women
10:00 am Elite Men & Wave One
10:20 am Wave Two
10:40 am Wave Three
Explosion happened somewhere around (edit) noon PST, 3pm EDT. A little before. Elite women marathon times are in the 2:15 and slower range. Elite men are 2:05 and slower. Not sure about likely times for the earlier groups.